You have to actually say that you are asserting your right (in the US) to stop interrogations.
There was a case recentlyish (you can search for details if you're interested, I can only recall the broad strokes) where an accused said "I want a lawyer, dawg" and this was interpreted as "I want a lawyer dog", as in a dog who is a lawyer, and this was not found to be an assertion of the right to remain silent. The whole thing was eye rollingly stupid, but when in America....
In order to get 99.9% certainty, 1) you are saying you are willing to have one in a thousand death penalties against innocents, and 2) that requires a system made of people to do their job correctly 99.9% of the time. I dont think there is a job on earth that people in a large group can do that well.